A wiring substrate includes insulation layers and wiring layers. The wiring layers are electrically connected to one another by vias that extend through the insulation layers. For example, a build-up process is performed to manufacture such a wiring substrate by alternately forming insulation layers and wiring layers one upon another. Each insulation layer includes via holes. The via holes are filled with a plating metal such as copper to form vias. Japanese Laid-Open Patent Publication No. 2001-291954 discloses a prior art wiring substrate.
In a wiring substrate, wiring patterns (wiring layers) have become finer. This has reduced the thickness of the wiring layers. Thus, when filling the via holes with copper, the amount of copper filling the via holes may be insufficient. In such a case, the insufficient amount of copper may lead to defective connection between the vias and the wiring layers.